Ultrathin films of metal hexacyanometalates were prepared upon multiple sequential
adsorption of metal cations M
m
+ (Fe3+, Fe2+, Co2+, and Ni2+) and hexacyanometalate anions
[M(CN)6]
n
- (Fe(CN)6
3-, Fe(CN)6
4-, and Co(CN)6
3-) on solid supports. The layer-by-layer
deposition led to the formation of films of the metal complex salts with monolayer precision.
The films were characterized using UV and IR spectroscopic methods and cyclic voltammetry.
The alternating adsorption of Fe3+ and Fe(CN)6
4- ions led to dense and defect-free films of
Prussian Blue, which were useful as membranes for ion separation. The porous, zeolitic
structure of Prussian Blue was permeable for ions with a small Stokes radius such as Cs+,
K+, and Cl-, whereas large hydrated ions such as Na+, Li+, Mg2+, or SO4
2- were blocked.
The effect of ion sieving increased with the thickness of the membrane. After a hundred
dipping cycles, high separation factors α(CsCl/NaCl) and α(KCl/NaCl) of 6.5 and 6.2,
respectively, were found. Corresponding membranes of cobalt and nickel hexacyanoferrate
were also useful for ion separation, but the α-values were lower. Possible reasons for the
differences in selectivity are discussed.